If you're like the majority of runners, you've experienced this at one point or another: You're out for a run, and then in creeps a sharp, localized pain, right under one of your ribs. You think, "What is that, and how to I banish it to hell forever?"

This may help.

Welcome to Fitmodo, Gizmodo's gym for your brain and backbone. Don't suffer through life as a sniveling, sickly weakling—brace up, man, get the blood pumping! Check back on Wednesdays for the latest in fitness science, workout gear, exercise techniques, and enough vim and vigor to whip you into shape.

What Is a "Runner's Stitch"?

It goes by many names: side stitch, runner's cramp, side ache, Beelzebub, but the medical name for it is "exercise-related transient abdominal pain," or ETAP. Despite it being one of the most common phenomenas in the exercise world (particularly common in runners and swimmers) there is little consensus as to what precisely it is and what causes it. There are, however, commonalities in most of the prevailing theories.

Virtually everyone agrees that it has to do with the diaphragm (the muscle, not the contraceptive device). The thoracic diaphragm is a sheet of muscle that extends along the bottom of the rib cage, and it's largely responsible for your breathing. As the diaphragm contracts it pulls air down into your lungs. As it relaxes, it pushes the air out. It is believed that the sensation of ETAP occurs when that muscle cramps (like your calf would) or becomes irritated.

What Causes It?

There are a few leading theories, but the answer most likely lies in a combination of them. One is that as a runner starts getting tired, his or her breathing is likely to get shorter and more shallow. Because your diaphragm is only fully relaxed when you've fully exhaled, those shallow breaths mean that the diaphragm is in a state of constant tension. Like any muscle, when it's hyper-exerted, it can cramp. This seems like the most likely theory to us.

A final popular theory is that it's caused by eating too much or too close to exercise. There isn't much in the way of hard science to back this up, the idea is that when your stomach is more expanded with food, it puts increased pressure on the organs and tissues around it. Combine with the aforementioned jiggle-factor and whammo, cramp.

What Cures It?

There's all manner of myth and sorcery and miracle cures here, including something about running with clumps of grass or pebbles in each of your hands. Silliness. That said, different methods work for different people. Here is what we have found to work best, both in our own experience and in our research.

1. As soon as you feel the cramp coming on, slow down a bit. This will allow you to take bigger, deeper, slower breaths. Inhale fully, and even more importantly, exhale fully. This will allow your diaphragm to relax more. Focus on bringing your breathing down lower, into your belly. This will also help oxygenate your blood. Raise your arm on the side that is cramping and gently stretch it as you run. If the pain keeps getting worse, slow down to a walk until it start to subside. This has been the most effective for us, and anecdotally, it seems to work best for others as well.

2. Another technique which you may already be doing instinctively is to press your hand into the place where it hurts. Generally this involves pushing your fingers or the palm of your hand slightly under you ribs and inward toward your center. Not only can this help by passively stretching your diaphragm, but it may help stabilize your innards, if all that bouncing around is indeed the cause.

3.Change up your breathing. This piggybacks on #1. If you are always exhaling when you land with your right foot, that could be wearing on you unevenly. Try breathing every third (or fifth) step, which will alternate the side you breathe on and will also help you slow things down a little. While you're at it, try taking smaller, smoother steps, with less bouncing. You don't have to slow down, you just have to quicken your cadence.

Last week you learned about the best headphones for running, and now you're going to hear what …
Read more Read more

4.Eat less before your run, or at least eat smarter. If you suspect that your full stomach is playing role in your cramping, look to eating more complex carbohydrates, which are primarily digested in your small intestine and thus have a faster gastric emptying rate (i.e. it leaves your stomach faster). Make sure you're properly hydrated, but not overly hydrated.

5. Lastly—and this is the good news—the more you run the more infrequent runner's stitches will become. Not only will you get more efficient with practice, but the muscles in your core will get stronger and stronger, including your diaphragm. You can further help this by doing core work when you aren't running. Pilates, for example, is really great for getting deep into your core, not just strengthening your rectus abdominis (your six-pack muscles on the outside). Not only will a stronger core be less susceptible to muscle fatigue (one of the probable causes of runner's stitch), it will help your guts not bounce around so much (another probable cause of runner's stitch).

Final Thoughts

Above all, use your common sense. This article is not a replacement for the advice of a doctor or physical therapist. If you have constant, nagging pain, get it checked out, as it could be something far more serious (a hernia, possible, or other un-fun things). Be safe and run smart. [National Institutes of Health, Body Results, About.com, RunAddicts]